Conventionally, there have been known various types of power supply devices as a power supply device in which multiple power devices operate in parallel and supply DC power to load devices.
As an example of such a conventional power supply device, a power supply device including two power devices in which output voltage monotonically decreases as output current increases is known (see, e.g., Japanese Patent Application Publication H10-248253). In the power supply device, the two power devices have different gradient angles in the output current-voltage characteristics. That is, when the output current changes by the same amount, an amount of change in the output voltage of one power device is different from that in the output voltage of the other power device.
In the above-described power supply device, each of the power devices is stabilized at a point where its output current-voltage characteristic is balanced with the load current, i.e., current supplied to the load devices. Therefore, a current and voltage outputted from each of the power device depend on the total amount of current (i.e., load current) consumed in the load devices.
Thus, in the power supply device in which the gradient angles of the output current-voltage characteristics of the two power devices are different from each other, the output voltage of each of the power devices, i.e., the voltage supplied to the load devices, varies on the magnitude of the load current. It is difficult to supply a stable voltage to the load devices. For example, when the output current of each power device is changed to a desired current value in the power supply device, it is necessary to horizontally move both the output current-voltage characteristics of the two power devices so as to maintain the voltage supplied to the load devices at a constant voltage before and after changing the output current, which makes the configuration complicated.
In order to solve such problem, a power supply device is considerable in which one of multiple power devices that operate in parallel is under constant voltage control and the others are under gradient control where DC voltage as output voltage monotonically decreases as output current increases. In this power supply device, the gradient-controlled power device outputs current to one or more load devices based on the output voltage of the gradient-controlled power device aligned with the output voltage (reference voltage) of the constant voltage-controlled power device.
In this case, a current corresponding to a deficiency in the load current is outputted to the load devices from the constant voltage-controlled power device. Accordingly, in this power supply device, even when the load current changes somewhat, the supply of power to the load devices can be stably performed while the voltage supplied to the load devices, i.e., the output voltage of a constant voltage-controlled power device is constantly maintained.
In the meantime, there is a case where a fuel cell is used as a power source that is connected to the power device described above. In this case, when a DC power supplied to the load devices from the fuel cell increases, a power supplied from a commercial-power-source power device using a commercial power source as an input power source, i.e., a power from an alternating current (AC) system can be decreased. By doing so, environment load can be reduced.
As for power generation using the fuel cell, hydrogen is used in generation and the hydrogen is produced from the town gas by a reformer. Thus, in order to change an amount of the power generated, it needs to perform a mechanical control such as adjustment of an amount of the town gas supplied to the reformer. There is a problem that a response to increase and decrease in the load current is lower in the power generation using the fuel cell than in the commercial power source.
Furthermore, when the load current has changed abruptly, the fuel cell operates to follow up the abrupt change in the load current, which leads to the problem of reduction in the durability of the fuel cells. For example, when the output of the fuel cells decreases abruptly, supply of the gas becomes excessive in the fuel cell and, accordingly, a generated voltage of the fuel cell increases. Then, the withstanding voltage property of a membrane for the fuel cell is deteriorated by the excessive voltage and therefore the durability of the fuel cells is reduced. On the other hand, when the output of the fuel cell increases abruptly, reaction in the fuel cell does not follow up the output, which may lead to reduction in the durability of the fuel cell.